Sterilization as a method of birth control for women appears more prone to fail than generally supposed, causing five to 20 times the expected number of unintended pregnancies, according to the first long-term, large-scale study of tubal ligation.

Long considered a quick, safe and seemingly permanent method of birth control, tubals have become one of the most popular and fastest-growing contraceptive choices. It is now more popular than the Pill among women older than 30.

The procedure -- in which a doctor surgically blocks the fallopian tubes that connect the ovaries to the uterus -- has long been rated one of the most effective methods of birth control short of hysterectomy or abstinence. Until now, though, the long-term success of tubal ligation had not been scrutinized.

"There has not been a really good long-term study before, and the studies that have been done are pretty far apart," said Dr. Herbert Peterson, lead author of the new study and chief of reproductive medicine at the federal Centers for Disease Control in Atlanta.

The overall failure rate for tubal ligation has been advertised to be as low as 0.1 percent, or just one unintended pregnancy per 1,000 women who undergo the operation. Planned Parenthood discloses a failure rate no higher than 0.5 percent, or just five pregnancies per 1,000 women.

In fact, the cumulative failure rate works out to roughly 2 percent, according to fertility experts who have been briefed about the new research results. Peterson will not comment on his study until it appears this week in the American Journal of Obstetrics and Gynecology.

Depending on how one looks at the new numbers, tubals seem not much more effective -- and possibly a bit less reliable -- than using birth control pills, an intrauterine device (IUD) or contraceptive implants.

Preliminary results of Peterson's study found 18.5 unintended pregnancies for every 1,000 operations, experts say. About 5.5 pregnancies per 1,000 were found to occur within a year. Women continued to be at risk for at least 10 years.

The study analyzed 10,685 women who had enrolled in the study between 1978 and 1986 and were tracked through 1994.

Younger women had the highest chance of becoming pregnant following a tubal, simply because they are apt to be fertile longer.

Failure rates, and the most likely time for a pregnancy, varies by type of procedure. For example, applying an electric current to sear shut the ends of the fallopian tubes appears to create the highest risk of failure within three years. Using special elastic bands to clamp the fallopian tubes may be the most reliable method in the long run.

The bottom line will not come as happy news for the 10 million U.S. women who have chosen sterilization as a method of birth control -- nearly one of every three women using a contraceptive.

"The most important finding," said Dr. Philip Darney, who participated in one phase of the 10-year study at San Francisco General Hospital, "is that some methods of sterilization have higher rates of failure in the long term than we expected."

Dr. David Adamson, director of Fertility Physicians of Northern California in Palo Alto and San Jose, said: "One would think that just blocking a tube is pretty easy, and most of the time it is. But it's not foolproof." In the United States, about 46 percent of married women over age 35 have had tubal ligations, according to the latest estimates. About 1.3 million tubals are performed each year, excluding sterilizations done when a woman has just given birth.

The surging popularity of tubal ligation came as new "minimally invasive" techniques were developed for the surgery. It is now typically an outpatient procedure that leaves almost no scars.

The good news is that researchers found no evidence of physical danger from the operation itself, despite the long-known risk of rare surgical complications. Nor did they uncover any major psychological hazards, although previous surveys have shown that about 1 percent of women who choose to be sterilized later regret the decision.

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Sterilization often can be surgically reversed, particularly in younger women, in an expensive procedure that nevertheless fails about a third of the time.

But what amazes some surgeons is the determined nature of the reproductive machinery in a large number of women.

"The body's desire to reproduce is very strong," said Cynthia Domecus, vice president for clinical research at Conceptus Inc., a San Carlos company that specializes in female reproductive technology. Domecus is the industry representative on a U.S. Food and Drug Administration obstetrics and gynecological advisory panel.

Domecus attended a meeting of the panel February 26 in Rockville, Md., where Peterson outlined his main findings. According to her detailed notes of the presentation, odds of failure varied with the surgical technique used. Tubals performed just after childbirth appeared the most reliable.

Some doctors said they hoped that the new study would prompt women to take a close look at all their options.

"Sterilization is a safe and effective method," Darney said. "But you have to assume it's not reversible -- and we now see it's not necessarily more effective than some of the temporary methods."